1. Field of the Invention
The present invention relates to a scanning module for reading a document image and outputting an electric signal representative of the document image and more particularly to a miniature, simple scanning module whose optical path length is easily adjustable.
2. Description of the Background Art
It is a common practice with a copier or similar image forming apparatus to use a scanning system of the type including a stationary line sensor and two carriages that move at full speed and half speed, respectively. This type of scanning system, conventional with an analog scanner, is not always desirable when it comes to the miniaturization and the reduction of production cost of a scanner.
A current trend in the imaging art is toward a scanner in which a line sensor, a lens, a lamp and so forth are constructed into a single movable scanning module in order to reduce production cost. The prerequisite with this type of scanner is that to focus imagewise reflection from a document on the line sensor in a preselected magnification, an optical path extending from the document to the line sensor be provided with a preselected length. This gives rise to a problem that the scanning module is bulky when provided with the preselected optical path length. While the scanning module may be reduced in size by use of a miniature lens having small optical path length, such a lens aggravates aberration and therefore prevents a beam from a document from being focused on the line sensor, resulting in low image quality.
In light of the above, it has been proposed to reduce the size of a scanning module by folding an optical path with, e.g., mirrors. Japanese Patent Laid-Open Publication No. 2001-174932, for example, discloses an image scanner including one or more multiple-reflection mirrors that reflect a beam from a document a plurality of times, so that necessary optical path length is implemented by a small number of mirrors (Prior Art 1 hereinafter).
Japanese Patent Laid-Open Publication No. 3-10564 teaches an image scanner including a transparent member configured to bend light incident from a document a plurality of times (Prior Art 2 hereinafter). Prior Art 2 implements necessary optical path length while reducing space necessary for reflected light.
Further, Japanese Patent Laid-Open Publication Nos. 7-170376 (Prior Art 3 hereinafter) and 9-163098 proposes an optical unit (Prior Art 4 hereinafter) each propose an optical unit including a pair of mirrors configured to reflect light incident from a document a plurality of times. Prior Arts 3 and 4 each contemplate to reduce the size of an optical unit while guaranteeing required optical path length.
Generally, scanners with reduction optical systems must be individually adjusted in optical path length between a focus on a document and a line sensor in order to accurately establish a designed optical magnification.
Prior Arts 1 through 4 described above have the following problems unsolved although contemplating to guarantee optical path length and reduce module size at the same time.
Prior Art 1 has a problem that optical path length cannot be adjusted unless a plurality of mirrors, including the multiple-reflection mirrors, are adjusted in position one by one, resulting in sophisticated adjustment. Further, in Prior Art 1, the distance between the mirror to which a beam from a document is incident first and a glass platen is selected to be smaller than the vertical distance between the optical axis of a lens and the glass platen. On the other hand, to accommodate great optical path length, the optical path between the document and the mirror to which the beam from the document is incident first should preferably have great length. In this respect, the structure taught in Prior Art 1 is not adequate from the size reduction standpoint.
As for Prior Art 2, it is likely that incident light or reflected light is refracted at the interface between the transparent member and the air due to a difference in refractive index. To obviate such refraction, light must be input to and output from the above interface perpendicularly to the interface. Therefore, when the position of the transparent member is varied, the mirrors, lens and image sensor must also be adjusted in position, resulting in sophisticated adjustment of optical path length. Further, because the transparent member bends incident light a plurality of times to thereby fold the optical path, it is difficult to reduce the size of the mirror to which light from a document is incident first and the size of the transparent member. This configuration is therefore not adequate from the size reduction standpoint.
Prior Art 3 does not allow optical path length to be easily adjusted because all the mirrors, including the pair of mirrors, must be individually varied in position. Moreover, light propagated through the lens crosses the optical path between the pair of mirrors, so that the pair of mirrors must be enclosed by a case capable of intercepting extraneous light, obstructing size and cost reduction of the module.
Prior Art 4 has the same problem as Prior Art 3 as to the adjustment of optical path length stated above. In addition, Prior Art 4 has another problem that the lens, having a small focal distance and used to reduce module size, needs high accuracy, obstructing cost reduction of the module.
As for the scanning system using a stationary line sensor and a full-speed and a half-speed carriage, as stated earlier, Japanese Patent Laid-Open Publication No. 11-146131 discloses an image scanner and an image forming apparatus configured to adjust optical path length by shifting only a mirror that reflects light toward reading means (Prior Art 5 hereinafter). However, Prior Art 5 is not applicable to a scanner of the type having a line sensor, lens, lamp and so forth constructed into a single movable module. Further, Prior Art 5 adjusts optical path length by moving a second mirror, which is originally movable in the scanning direction, i.e., without using any exclusive member for the adjustment. That is, Prior Art 5 simply controls the movement of the half-speed carriage in the conventional scanner.
Japanese Patent Laid-Open Publication No. 7-56242 teaches an image scanning unit of the type folding an optical path by reflecting light from a document with, e.g., mirrors (Prior Art 6 hereinafter). To facilitate the adjustment of optical path length, Prior Art 6 uses a lens and an image pickup section constructed integrally with each other and movable along an optical path. This, according to the above document, allows magnification to be easily adjusted.
Prior Art 6, however, needs an exclusive mechanism for moving only the lens along the optical axis and an exclusive mechanism for moving a lens unit, including the lens and image pickup section, along the optical axis. Further, Prior Art 6 needs a mechanism for affixing the lens and lens unit at desired positions. A module with such mechanisms is sophisticated in structure and cannot be reduced in size. Moreover, the adjustment of optical path length effected by moving the lens and image pickup section together needs a large margin for adjustment because the amount of displacement and the amount of variation of optical path length are equal to each other.
As stated above, as for a scanning system of the type including a line sensor, a lens, a lamp and so forth constructed into a single movable module, none of Prior Arts 1 through 6 realizes a simple, compact module allowing its optical path length to be easily adjusted.